Baltimore
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Baltimore clear sky chart from 2005-11-06 to 2024-03-28.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 6% 2 5% 5% 3 5% 5% 6% 8% 42%	78794
	Night	15% 6% 2 5% 5% 3 4 4 5% 7% 44%	81204
Transparency	Day	40% 6% 12% 23% 16% 3	78504
	Night	40% 5% 11% 24% 17% 3	81014
Seeing	Day	19% 13% 28% 24% 12% 4	14110
	Night	18% 12% 29% 28% 12% 1	23695

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	17% 5% 1 4 3 3 4 4 5% 6% 48%	2517
	11th to 20th	Day	13% 11% 4 6% 5% 2 4 4 5% 5% 40%	2812
	21st to end	Day	19% 7% 3 6% 5% 4 4 4 5% 6% 38%	3124
February	1st to 10th	Day	15% 5% 2 6% 6% 3 5% 5% 5% 7% 40%	1710
	11th to 20th	Day	18% 7% 3 4 4 2 4 4 6% 6% 42%	1743
	21st to end	Day	19% 7% 2 5% 5% 3 4 4 5% 5% 39%	1570
March	1st to 10th	Day	14% 5% 3 4 4 3 4 3 4 7% 48%	2004
	11th to 20th	Day	15% 6% 2 5% 5% 3 4 5% 5% 7% 42%	2090
	21st to end	Day	18% 7% 3 6% 6% 3 5% 4 4 6% 39%	2317
April	1st to 10th	Day	13% 5% 2 4 5% 3 5% 6% 6% 7% 43%	2160
	11th to 20th	Day	13% 5% 2 5% 5% 2 5% 4 5% 7% 46%	2134
	21st to end	Day	19% 5% 3 4 4 3 4 4 6% 8% 40%	2160
May	1st to 10th	Day	17% 6% 2 5% 5% 3 4 5% 6% 8% 38%	2265
	11th to 20th	Day	20% 6% 2 4 4 4 5% 5% 6% 8% 36%	2468
	21st to end	Day	14% 5% 2 5% 5% 3 5% 5% 6% 9% 40%	2725
June	1st to 10th	Day	12% 4 3 5% 4 3 5% 5% 7% 11% 41%	2520
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 5% 6% 8% 41%	2520
	21st to end	Day	10% 4 1 4 5% 3 6% 6% 7% 11% 43%	2520
July	1st to 10th	Day	9% 4 2 5% 6% 4 5% 6% 7% 8% 43%	2520
	11th to 20th	Day	5% 3 2 4 5% 4 6% 7% 9% 10% 45%	2519
	21st to end	Day	9% 5% 2 6% 6% 3 6% 7% 8% 10% 37%	2547
August	1st to 10th	Day	10% 4 3 6% 7% 3 6% 7% 7% 9% 37%	2327
	11th to 20th	Day	11% 5% 2 5% 5% 4 5% 7% 7% 9% 39%	2184
	21st to end	Day	12% 5% 2 5% 5% 4 6% 6% 7% 9% 41%	2376
September	1st to 10th	Day	14% 6% 2 5% 4 3 5% 4 7% 7% 44%	2160
	11th to 20th	Day	11% 5% 2 5% 4 3 4 4 5% 9% 48%	2160
	21st to end	Day	18% 6% 3 5% 4 3 5% 4 5% 7% 39%	1913
October	1st to 10th	Day	17% 5% 2 5% 4 3 3 4 4 7% 47%	1800
	11th to 20th	Day	18% 4 2 4 3 2 3 4 6% 6% 48%	1800
	21st to end	Day	16% 5% 2 4 4 3 4 4 4 5% 48%	1980
November	1st to 10th	Day	13% 6% 2 3 3 2 3 4 5% 7% 51%	1709
	11th to 20th	Day	15% 6% 3 3 3 2 4 4 5% 6% 50%	1710
	21st to end	Day	15% 5% 2 4 4 3 4 4 5% 5% 48%	1528
December	1st to 10th	Day	18% 6% 2 5% 3 2 4 5% 5% 8% 42%	1520
	11th to 20th	Day	17% 6% 3 4 4 2 4 4 4 8% 44%	1910
	21st to end	Day	17% 6% 2 6% 5% 4 4 5% 5% 7% 38%	2772
January	1st to 10th	Night	16% 6% 3 6% 5% 3 3 4 5% 6% 44%	4429
	11th to 20th	Night	19% 7% 2 4 4 2 3 3 4 7% 45%	4147
	21st to end	Night	21% 9% 2 5% 4 2 3 3 4 5% 41%	4532
February	1st to 10th	Night	17% 7% 2 5% 4 3 4 4 6% 6% 41%	2470
	11th to 20th	Night	18% 7% 3 5% 4 2 4 4 5% 6% 44%	2321
	21st to end	Night	17% 7% 2 4 4 3 4 4 4 6% 43%	1884
March	1st to 10th	Night	15% 5% 2 4 4 2 4 4 5% 6% 49%	2171
	11th to 20th	Night	15% 7% 2 5% 5% 2 4 4 6% 6% 45%	2090
	21st to end	Night	15% 7% 3 6% 5% 3 4 4 4 6% 43%	2194
April	1st to 10th	Night	15% 6% 3 5% 5% 3 4 5% 6% 6% 42%	1800
	11th to 20th	Night	14% 5% 2 4 5% 3 4 4 5% 5% 47%	1775
	21st to end	Night	19% 6% 1 4 4 3 4 4 5% 7% 44%	1800
May	1st to 10th	Night	18% 6% 2 5% 5% 3 5% 4 5% 8% 39%	1710
	11th to 20th	Night	19% 7% 3 5% 4 3 4 4 6% 7% 39%	1423
	21st to end	Night	15% 5% 1 5% 5% 3 5% 4 6% 11% 41%	1550
June	1st to 10th	Night	12% 4 2 4 4 3 4 4 7% 11% 45%	1440
	11th to 20th	Night	11% 5% 2 6% 5% 3 5% 6% 6% 10% 40%	1440
	21st to end	Night	11% 4 2 4 5% 3 6% 6% 6% 11% 42%	1440
July	1st to 10th	Night	9% 5% 3 7% 6% 4 7% 6% 7% 9% 37%	1440
	11th to 20th	Night	6% 3 3 4 6% 4 5% 6% 7% 11% 45%	1440
	21st to end	Night	7% 5% 3 5% 4 4 5% 6% 7% 12% 43%	1768
August	1st to 10th	Night	9% 5% 2 4 6% 3 6% 7% 7% 10% 39%	1611
	11th to 20th	Night	11% 6% 3 6% 7% 4 5% 5% 7% 7% 40%	1696
	21st to end	Night	13% 5% 2 5% 4 3 5% 5% 6% 9% 43%	1980
September	1st to 10th	Night	14% 7% 3 5% 3 3 4 4 6% 7% 45%	1800
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 4 7% 52%	1800
	21st to end	Night	19% 6% 3 5% 4 3 3 5% 5% 8% 40%	2042
October	1st to 10th	Night	20% 6% 2 4 5% 3 3 4 5% 5% 44%	2160
	11th to 20th	Night	16% 4 2 4 4 3 3 4 4 6% 50%	2160
	21st to end	Night	16% 5% 2 4 4 3 3 3 4 7% 48%	2376
November	1st to 10th	Night	12% 5% 2 4 4 2 4 3 4 6% 53%	2366
	11th to 20th	Night	15% 6% 3 4 4 2 3 3 4 5% 50%	2470
	21st to end	Night	13% 4 2 4 5% 3 4 4 5% 7% 48%	2650
December	1st to 10th	Night	17% 6% 2 5% 5% 3 4 4 5% 6% 41%	2660
	11th to 20th	Night	17% 5% 2 5% 5% 3 4 4 4 7% 44%	3285
	21st to end	Night	16% 6% 2 5% 5% 3 5% 6% 6% 8% 37%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 4 9% 31% 15% 4	2518
	11th to 20th	Day	47% 6% 9% 20% 14% 4	2812
	21st to end	Day	48% 6% 12% 22% 11% 2	3124
February	1st to 10th	Day	42% 5% 14% 24% 13% 2	1710
	11th to 20th	Day	41% 4 10% 26% 15% 3	1752
	21st to end	Day	46% 5% 9% 21% 14% 4	1570
March	1st to 10th	Day	37% 5% 10% 27% 18% 3	2004
	11th to 20th	Day	41% 6% 10% 19% 19% 6%	2090
	21st to end	Day	47% 5% 8% 19% 18% 3	2317
April	1st to 10th	Day	39% 6% 12% 19% 18% 5%	2160
	11th to 20th	Day	37% 6% 10% 22% 18% 6%	2134
	21st to end	Day	41% 5% 12% 20% 19% 4	2160
May	1st to 10th	Day	43% 7% 11% 20% 16% 3	2265
	11th to 20th	Day	44% 7% 13% 19% 13% 3	2472
	21st to end	Day	40% 7% 15% 24% 13% 2	2725
June	1st to 10th	Day	38% 8% 14% 24% 15% 2	2520
	11th to 20th	Day	40% 8% 12% 23% 15% 3	2520
	21st to end	Day	34% 7% 15% 25% 18% 1	2520
July	1st to 10th	Day	36% 9% 14% 22% 17% 3	2450
	11th to 20th	Day	29% 10% 16% 28% 15% 3	2444
	21st to end	Day	38% 10% 14% 21% 16% 2	2405
August	1st to 10th	Day	39% 9% 13% 25% 13% 1	2314
	11th to 20th	Day	37% 9% 14% 22% 15% 2	2184
	21st to end	Day	37% 8% 13% 23% 15% 4	2376
September	1st to 10th	Day	39% 6% 12% 20% 18% 4	2160
	11th to 20th	Day	34% 5% 10% 26% 22% 4	2160
	21st to end	Day	44% 5% 7% 22% 19% 2	1913
October	1st to 10th	Day	39% 5% 11% 24% 19% 3	1800
	11th to 20th	Day	37% 5% 11% 23% 22% 3	1800
	21st to end	Day	39% 5% 8% 28% 19% 2	1980
November	1st to 10th	Day	33% 4 13% 26% 20% 4	1709
	11th to 20th	Day	36% 5% 10% 23% 24% 4	1710
	21st to end	Day	39% 4 11% 29% 16% 2	1528
December	1st to 10th	Day	41% 5% 15% 24% 13% 2	1520
	11th to 20th	Day	40% 5% 10% 31% 12% 2	1906
	21st to end	Day	45% 7% 14% 22% 12% 1	2772
January	1st to 10th	Night	41% 4 9% 27% 15% 3	4429
	11th to 20th	Night	41% 4 10% 26% 17% 2	4147
	21st to end	Night	46% 4 7% 28% 13% 2	4532
February	1st to 10th	Night	44% 4 12% 24% 14% 2	2470
	11th to 20th	Night	43% 3 9% 22% 18% 5%	2323
	21st to end	Night	43% 4 10% 21% 17% 5%	1884
March	1st to 10th	Night	36% 4 11% 27% 18% 3	2171
	11th to 20th	Night	39% 6% 10% 22% 18% 4	2090
	21st to end	Night	42% 4 10% 19% 18% 6%	2194
April	1st to 10th	Night	42% 5% 10% 19% 20% 4	1800
	11th to 20th	Night	38% 5% 11% 21% 20% 6%	1775
	21st to end	Night	40% 6% 10% 22% 17% 6%	1800
May	1st to 10th	Night	43% 6% 11% 21% 15% 4	1710
	11th to 20th	Night	46% 6% 11% 15% 18% 3	1424
	21st to end	Night	37% 9% 17% 22% 12% 3	1551
June	1st to 10th	Night	35% 8% 11% 27% 18% 2	1440
	11th to 20th	Night	39% 8% 13% 23% 15% 2	1440
	21st to end	Night	35% 6% 13% 25% 18% 3	1440
July	1st to 10th	Night	40% 10% 16% 18% 13% 4	1400
	11th to 20th	Night	30% 11% 15% 25% 16% 3	1392
	21st to end	Night	31% 9% 16% 25% 18% 1	1670
August	1st to 10th	Night	36% 9% 16% 22% 15% 2	1600
	11th to 20th	Night	41% 8% 13% 21% 15% 2	1696
	21st to end	Night	37% 7% 13% 24% 15% 4	1980
September	1st to 10th	Night	38% 5% 11% 23% 18% 5%	1800
	11th to 20th	Night	33% 4 10% 26% 23% 4	1800
	21st to end	Night	42% 6% 8% 21% 20% 3	2042
October	1st to 10th	Night	43% 5% 10% 22% 18% 3	2160
	11th to 20th	Night	36% 4 9% 29% 20% 3	2160
	21st to end	Night	38% 4 9% 26% 21% 3	2376
November	1st to 10th	Night	34% 4 9% 26% 21% 6%	2366
	11th to 20th	Night	37% 4 9% 23% 21% 6%	2470
	21st to end	Night	36% 5% 11% 26% 20% 3	2650
December	1st to 10th	Night	44% 5% 11% 24% 15% 1	2660
	11th to 20th	Night	41% 4 11% 22% 19% 4	3288
	21st to end	Night	42% 8% 14% 24% 11% 1	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	27% 24% 33% 12% 4 1	180
	11th to 20th	Day	27% 29% 29% 14% 2	263
	21st to end	Day	19% 31% 31% 14% 4 1	297
February	1st to 10th	Day	25% 20% 39% 14% 2	270
	11th to 20th	Day	32% 22% 37% 8% 1	350
	21st to end	Day	28% 21% 31% 16% 3	382
March	1st to 10th	Day	23% 25% 31% 18% 3	460
	11th to 20th	Day	28% 21% 30% 15% 4 1	460
	21st to end	Day	25% 20% 33% 16% 4 1	490
April	1st to 10th	Day	21% 23% 35% 16% 4 2	420
	11th to 20th	Day	19% 18% 32% 19% 9% 3	412
	21st to end	Day	24% 17% 36% 15% 6% 1	420
May	1st to 10th	Day	28% 16% 29% 16% 9% 2	423
	11th to 20th	Day	22% 13% 31% 23% 8% 2	447
	21st to end	Day	18% 6% 25% 28% 18% 4	485
June	1st to 10th	Day	14% 10% 29% 28% 14% 6%	450
	11th to 20th	Day	17% 7% 26% 31% 15% 4	450
	21st to end	Day	12% 9% 26% 32% 15% 5%	450
July	1st to 10th	Day	12% 3 19% 32% 24% 10%	445
	11th to 20th	Day	5% 2 21% 38% 24% 9%	444
	21st to end	Day	14% 3 27% 29% 21% 6%	488
August	1st to 10th	Day	11% 4 20% 35% 20% 9%	446
	11th to 20th	Day	13% 3 18% 36% 21% 10%	439
	21st to end	Day	11% 4 25% 31% 19% 11%	495
September	1st to 10th	Day	17% 7% 18% 27% 19% 12%	450
	11th to 20th	Day	14% 6% 20% 34% 18% 9%	450
	21st to end	Day	24% 10% 22% 29% 11% 4	450
October	1st to 10th	Day	23% 8% 23% 23% 18% 5%	450
	11th to 20th	Day	18% 13% 29% 24% 15% 2	450
	21st to end	Day	19% 14% 37% 21% 7% 2	495
November	1st to 10th	Day	16% 12% 35% 23% 10% 3	383
	11th to 20th	Day	18% 18% 33% 21% 9% 1	370
	21st to end	Day	22% 19% 32% 20% 8%	188
December	1st to 10th	Day	23% 23% 33% 17% 3 1	180
	11th to 20th	Day	27% 19% 34% 15% 4	180
	21st to end	Day	24% 18% 33% 20% 6%	198
January	1st to 10th	Night	20% 18% 38% 22% 3	760
	11th to 20th	Night	24% 16% 32% 23% 5%	760
	21st to end	Night	19% 19% 38% 22% 2	836
February	1st to 10th	Night	20% 15% 35% 27% 3	760
	11th to 20th	Night	24% 20% 36% 18% 1	743
	21st to end	Night	24% 19% 35% 19% 3	628
March	1st to 10th	Night	20% 17% 36% 24% 4	760
	11th to 20th	Night	21% 19% 33% 20% 6% 1	760
	21st to end	Night	20% 14% 39% 21% 5% 1	821
April	1st to 10th	Night	21% 20% 35% 19% 5%	720
	11th to 20th	Night	17% 15% 33% 27% 8%	711
	21st to end	Night	23% 13% 32% 26% 7%	720
May	1st to 10th	Night	22% 9% 30% 30% 8% 1	630
	11th to 20th	Night	22% 11% 27% 30% 11%	362
	21st to end	Night	17% 6% 19% 36% 19% 2	389
June	1st to 10th	Night	15% 3 25% 38% 19% 1	360
	11th to 20th	Night	11% 5% 23% 40% 20% 1	360
	21st to end	Night	11% 3 25% 46% 16%	360
July	1st to 10th	Night	10% 5% 18% 38% 27% 3	350
	11th to 20th	Night	8% 3 14% 41% 32% 3	348
	21st to end	Night	9% 3 18% 40% 27% 3	588
August	1st to 10th	Night	11% 2 13% 42% 28% 3	537
	11th to 20th	Night	14% 5% 13% 36% 28% 4	632
	21st to end	Night	14% 3 17% 35% 26% 4	792
September	1st to 10th	Night	17% 3 21% 32% 24% 4	720
	11th to 20th	Night	13% 3 16% 36% 27% 5%	720
	21st to end	Night	22% 4 20% 33% 19% 2	720
October	1st to 10th	Night	23% 6% 20% 28% 18% 5%	720
	11th to 20th	Night	19% 8% 27% 32% 14% 1	720
	21st to end	Night	20% 12% 30% 25% 12% 1	792
November	1st to 10th	Night	15% 10% 33% 30% 10% 2	740
	11th to 20th	Night	18% 17% 28% 27% 8% 1	760
	21st to end	Night	16% 17% 31% 29% 7% 1	760
December	1st to 10th	Night	22% 14% 34% 21% 7% 1	760
	11th to 20th	Night	17% 15% 38% 23% 7%	760
	21st to end	Night	20% 15% 34% 29% 3	836

Page generated at 2024-03-24 23:37:01. Version ac07

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